Vietri and Stella
[426]
suggested that GRBs take place when a
"supermassive" (or supramassive as
Vietri and Stella
[426]
call it) neutron star (namely a neutron star that is above the maximal
cold nonrotating neutron star mass) collapses to a black hole. The
collapse can take place because the neutron star losses angular
momentum via a pulsar wind and it looses the extra support of the
centrifugal force. Alternatively the supramassive neutron star can
simply cool and become unstable if rotation alone is not enough to
support it. The neutron star could also become over massive and
collapse if it accretes slowly matter from a surrounding accretion
disk [427].
In this latter case the time delay
from the SN could be very large and the SNR will not play any role
in the GRB or its afterglow.

The Supranova model is a two step event. First, there is a
supernova, which may be more energetic than an average one, in
which the supermassive neutron star forms. Then a few weeks or
months later this neutron star collapses producing the GRB. While
both the Supranova and the Collapsar (or hypernova) events are
associated with Supernovae or Supernovae like event the details of
the model are very different. First, while in the Collapsar model
one expect a supernova bump on the afterglow light curve, such a
bump is not expected in the Supranova model unless the time delay
is a few days. On the other hand while it is not clear in the
Collapsar model how does the Fe needed for the Fe X-ray lines reach
the implied large distances form the center, it is obvious in
this model, as the supernova shell was ejected to space several
month before the GRB. As mentioned earlier (see
Section IIC4) the association of GRB 030329
with SN 2003dh
[167,
395]
is incompatible with the Supranova
model. Proponents of this model, argue however, that there might
be a distribution of delay times between the first and second collapses.

The models are also very different in their physical content.
First in the Supranova model the GRB jet does not have to punch a
whole through the stellar envelope. Instead the ejecta propagates
in almost free space polluted possibly by a pulsar wind
[149,
183].
In both models, like in many
other models, the GRB is powered by accretion of a massive
accretion disk surrounding the newborn black hole. This accretion
disk forms, from the debris of the collapsing neutron star at the
same time that the black hole is formed. Again, the time scale of
the burst is determined by the accretion time of this disk.
Narayan et al.
[277]
(see also Section IXA)
point however that long lived (50 sec) accretion disks must be
large and hence extremely inefficient. This may pose a problem
for this model.

Königl and Granot
[183],
Guetta and Granot
[149] and
Inoue et al.
[178]
considered the effects a strong pulsar wind
(that may exist after the SN and before the second collapse) on
this scenario. The pulsar wind can have several effects. First it
would produce a denser highly magnetized medium into which the GRB
jet propagates. The strong magnetic field will be amplified by the
afterglow shock. This resolves the problem of the source of the
strong magnetic field needed for the synchrotron afterglow model.
This can also explain the high energy emission detected by EGRET
in GRB 940217 (Hurley
[172] and
Section IIA1) by
Inverse Compton scattering on the pulsar wind bubble photons. On
the other hand the density of this wind matter (~ 103
cm-3) might be too high for the spherical model. Note
however, that this depends on the time delay as
t-3. However,
the pulsar wind won't be spherical and one would expect that it
will form an elongated supernova shell cavity within which the
pulsar wind is bounded. If, as expected, the pulsar jet coincides
with the GRB jet then the relativistic ejecta will move along the
elongated direction of this shell.